Automated speed ramp control of stepper motors

ABSTRACT

Automated speed ramp control of stepper motor acceleration and deceleration using direct memory access (DMA) and core independent peripherals (CIPs) comprises a numerically controlled oscillator (NCO) controlled through direct memory access (DMA) transfers of prescale values used in combination with a clock oscillator to generate clock pulses that are a function of the clock oscillator frequency and the prescale values. This automates changing the frequency of the NCO, thereby controlling steeper motor speed, without requiring computer processing unit (CPU) overhead. The DMA module is enabled during a first number of clock pulses for step speed acceleration, disabled during a second number of clock pulses for normal operation at full step speed, and then re-enabled during a third number of clock pulses for step speed deceleration. A table in memory may store and provide a plurality of acceleration and deceleration prescale values for DMA transfers to the NCO.

RELATED PATENT APPLICATION

This application claims priority to commonly owned U.S. ProvisionalPatent Application Ser. No. 62/750,434; filed Oct. 25, 2018; entitled“Automated Speed Ramp Control for Stepper Motors,” and U.S.Non-Provisional patent application Ser. No. 16/254,797 filed Jan. 23,2019 entitled “Automated Speed Ramp Control Of Stepper Motors” by KeithEdwin Curtis; and are hereby incorporated by reference herein for allpurposes.

TECHNICAL FIELD

The present disclosure relates to speed control for stepper motors and,more particularly, to a method and apparatus for automated speed rampcontrol of stepper motors.

BACKGROUND

Stepper motors are synchronous positioning motors. To run at high steprates the stepper motor must be accelerated and decelerated (step speedramp control) to retain lock. By using a numerically controlledoscillator (NCO) based stepper circuit the step speed sequencing may beautomated. However, this requires software overhead to accelerate anddecelerate the stepper motor(s). When the stepper motor speed isincreased or decreased, software running on a central processing unit(CPU) of a microcomputer or microcontroller must provide new prescalerNCO values and load them into the prescaler NCO. This requires a highpriority interrupt to the CPU, thereby reducing performance of othersoftware related functions.

SUMMARY

Therefore, what is needed is an automated stepper motor acceleration anddeceleration (speed ramp control) method and apparatus using directmemory access (DMA) to reduce the core processing, e.g., centralprocessing unit (CPU), loading and improve software instructionexecution timing thereof.

According to an embodiment, a method for speed ramp control of a steppermotor using direct memory access (DMA) transfers may comprise the stepsof: providing a numerically controlled oscillator (NCO) havingoscillator and prescale value inputs and an output providing clockpulses, wherein a frequency of the clock pulses may be a function of theoscillator frequency and the prescale value; providing a table ofacceleration and deceleration prescale values in a memory; programmingthe NCO with the acceleration prescale values from the memory usingdirect memory access (DMA) transfers for a first number of clock pulses;inhibiting DMA transfers to the NCO for a second number of clock pulses;and programming the NCO with the deceleration prescale values from thememory using the DMA transfers for a third number of clock pulses.

According to a further embodiment of the method, may comprise the stepof counting a certain number of clock pulses from the NCO before doingthe DMA transfer to the NCO. According to a further embodiment of themethod, may comprise the step of counting the number of clock pulsesfrom the NCO then inhibiting the output of the NCO after the number ofclock pulses reaches a maximum count.

According to another embodiment, an apparatus for automated speed rampcontrol of stepper motor acceleration and deceleration may comprise: anoscillator having an output; a numerically controlled oscillator (NCO)having oscillator and prescale value inputs and an output providingclock pulses, wherein the NCO oscillator input may be coupled to theoscillator output and a frequency of the clock pulses from the NCO maybe a function of a frequency from the oscillator and the prescale value;a memory for storing a plurality of acceleration and decelerationprescale values; and a direct memory access (DMA) control circuitcoupled between the NCO prescale value input and the memory, and theoutput of the NCO, wherein the DMA control circuit may provide the NCOwith the acceleration prescale values from the memory for a first numberof clock pulses, inhibits the DMA control circuit for a second number ofclock pulses, and then may provide the NCO with the decelerationprescale values from the memory for a third number of clock pulses.

According to a further embodiment, an accumulator counter may be coupledbetween the output of the NCO and the DMA control circuit, wherein theDMA control circuit may provide a single prescale value to the NCO aftera certain number of clock pulses have occurred. According to a furtherembodiment a maximum counter may be coupled to the output of the NCO andinhibiting operation of the NCO after receiving a certain maximum numberof clock pulses therefrom.

According to a further embodiment, the DMA control circuit may comprise:a pulse width modulation (PWM) module having an input coupled to theclock output of the NCO and an output; and a data signal modulator (DSM)having a first input coupled to the clock output of the NCO, a secondinput coupled to a fixed logic level, an output coupled to a DMA triggerinput of the DMA module and a control input coupled to the PWM moduleoutput for selectively coupling the output of the DSM between the firstand second inputs thereof, wherein the first input may be selected whenthe output of the PWM module may be at a first logic level and thesecond input may be selected when the output of the PWM module may be ata second logic level.

According to a further embodiment, an accumulator counter may be coupledbetween the output of the NCO, and the first input of the DSM and theinput of the PWM module, wherein the DMA module may provide a singleprescale value to the NCO after a certain number of clock pulses haveoccurred. According to a further embodiment, a duty cycle of the PWMmodule may be programmable to provide the first logic level outputduring the first number of clock pulses, the second logic level outputduring the second number of clock pulses, and the first logic leveloutput during the third number of clock pulses. According to a furtherembodiment, the first logic level may be a logic zero and the secondlogic level may be a logic one. According to a further embodiment, thefirst logic level may be a logic one and the second logic level may be alogic zero. According to a further embodiment, the output of the DSM maybe configured to switch between the first and second inputs thereof onan edge of a control signal to the control input of the DSM.

According to a further embodiment, the DMA control circuit may comprise:a first counter for counting the first number of clock pulses; a secondcounter for counting the second number of clock pulses; a third counterfor counting the third number of clock pulses; and a multiplexer havinga first input coupled to the NCO output, a second input coupled to afixed logic level, an output coupled to the DMA module and inputselection controlled by the first, second and third counters; whereinthe first counter controls the multiplexer to couple the first input tothe output thereof during the first number of clock pulses, the secondcounter controls the multiplexer to couple the second input to theoutput thereof during the second number of clock pulses, and the thirdcounter controls the multiplexer to couple the first input to the outputthereof during the third number of clock pulses.

According to yet another embodiment, an apparatus for automated speedramp control of stepper motor acceleration and deceleration maycomprise: an oscillator having an output; a numerically controlledoscillator (NCO) having oscillator and prescale value inputs and anoutput providing clock pulses, wherein the NCO oscillator input may becoupled to the oscillator output and a frequency of the clock pulses maybe a function of a frequency from the oscillator and the prescale value;a memory for storing a plurality of acceleration and decelerationprescale values; a direct memory access (DMA) module coupled between theNCO prescale value input and the memory; a pulse width modulation (PWM)module having an input coupled to the clock output of the NCO; and adata signal modulator (DSM) having a first input coupled to the clockoutput of the NCO, a second input coupled to a fixed logic level, anoutput coupled to a DMA trigger input of the DMA module and a controlinput for selectively coupling the output of the DSM between the firstand second inputs thereof; wherein the DMA module programs the NCO withthe acceleration prescale values during a first number of clock pulses,does no programming of the NCO during a second number of clock pulses,and programs the NCO with the deceleration prescale values during athird number of clock pulses.

According to a further embodiment, an accumulator counter may be coupledbetween the clock output of the NCO, and the first input of the DSM andthe input of the PWM module, wherein the DMA module may provide a singleprescale value to the NCO after a certain number of clock pulses haveoccurred. According to a further embodiment, a maximum counter may becoupled to the output of the NCO and inhibiting operation of the NCOafter receiving a certain maximum number of clock pulses therefrom.According to a further embodiment, a duty cycle of the PWM module may beprogrammable to provide the first logic level output during the firstnumber of clock pulses, the second logic level output during the secondnumber of clock pulses, and the first logic level output during thethird number of clock pulses. According to a further embodiment, theoutput of the DSM may be configured to switch between the first andsecond inputs thereof on an edge of a control signal to the controlinput of the DSM.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure may be acquiredby referring to the following description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 illustrates a schematic block diagram of a three-axis steppermotor circuit, according to the teachings of this disclosure;

FIGS. 2, 3 and 4 illustrate schematic block diagrams of automated speedramp control circuits using direct memory access (DMA) transfers to loadspeed ramp control values for acceleration and deceleration of steppermotors, according to specific example embodiments of this disclosure;

FIG. 5 illustrates schematic graphs of the operation of the automatedspeed ramp control circuits shown in FIGS. 2, 3 and 4; and

FIG. 6 illustrates a schematic block diagram of functional circuitvariations of the automated speed ramp control circuits shown in FIGS.2, 3 and 4.

While the present disclosure is susceptible to various modifications andalternative forms, specific example embodiments thereof have been shownin the drawings and are herein described in detail. It should beunderstood, however, that the description herein of specific exampleembodiments is not intended to limit the disclosure to the formsdisclosed herein.

DETAILED DESCRIPTION

Embodiments of the present disclosure may provide automated speed rampcontrol for acceleration and deceleration of stepper motors using directmemory access (DMA) transfers to load the speed ramp control valueswithout requiring intervention or program execution by an integratedcircuit digital processor, e.g., central processing unit (CPU) of, forexample but not limited to, a microcontroller unit (MCU) to maintainoperation of acceleration and deceleration of the stepper motor(s). Inaddition, core independent peripherals (CIPs) may be used to facilitatethe design of the automated stepper motor acceleration and deceleration(speed ramp control) circuit. CIPs are functional module blocks designedto handle their tasks with no software code or supervision from a CPU.Because the CIP functionality is performed in hardware instead ofsoftware, CIPs enable circuit performance that is far beyond thecapabilities of typical microcontroller units (MCUs) while simplifyingthe design experience and reducing memory cost. Designers can createcomplex control circuits while having the flexibility to more easilyinnovate their embedded solutions. This functionality is especiallyadvantage for use in computer numerical control (CNC) machines.

According to specific example embodiments of this disclosure, automatedstepper motor acceleration and deceleration operational information(output clock frequency which determines the step speed) may be providedby a numerically controlled oscillator (NCO). The NCO is a timer thatuses the overflow from the addition of an increment value to divide asignal having an input frequency by a divisor value to output a signalhaving a frequency of the input signal divided by the divisor value. Theadvantage of using this addition method over a simple counter driventimer or a pulse width modulator (PWM) is that the resolution ofdivision does not vary with the divider value, thereby resulting in truelinear frequency control. The NCO is most useful for applications thatrequire linear frequency control, high frequency accuracy and fineresolution at a fixed duty cycle.

According to the teachings of this disclosure, the NCO may be configuredas a frequency prescaler (hereinafter referred to as a “prescaler NCO”)and having frequency prescale data provided through direct memory access(DMA) transfers without requiring intervention or program execution bythe CPU of the MCU. This automates changing of the frequency prescaledata into the prescaler NCO. A DMA control circuit may be used tocontrol enabling and disabling DMA transfers, wherein DMA transfers areenabled during step speed acceleration and deceleration, and disabledduring normal running step speed. A table in memory may be provided forstep speed acceleration and deceleration frequency prescale data(determines clock frequencies—step speeds) whereby DMA suspension is allthat is needed between the end of the step speed acceleration and thestart of step speed deceleration to build a complete step speedacceleration/deceleration profile.

According to specific example embodiments of this disclosure, CPUprogram software intervention may be eliminated from the stepper motoracceleration and deceleration process. The acceleration/decelerationcurve (determines clock pulse speed profile) may be loaded into amemory, and then stepper motor operation may be initiated. The circuitautomatically ramps up (accelerates the step speeds) for a certain firstnumber of NCO frequency cycles (clock pulses), then runs at the highestNCO frequency based upon the last frequency prescale data DMAtransferred into the NCO during the acceleration phase for a certainsecond number of NCO frequency cycles, and then starts the decelerationphase (slows down step speed) for a certain third number of NCOfrequency cycles. When the acceleration/deceleration curve hascompleted, the automated stepper motor acceleration and decelerationcircuit may provide an interrupt to trigger software loading of the nextstepper motor(s) movement curve (profile), e.g., from the CPU. Anacceleration and deceleration curve (profile) is again started and theCNC machine continues operation.

An advantage of the automated stepper motor acceleration anddeceleration circuit is that the acceleration and deceleration of theCNC clock is completely automated without intervention from the CPU ofthe CNC circuit.

An acceleration/deceleration table may be programmed into, for examplebut is not limited to, flash memory. A DMA CIP may be used to automateoperation of the acceleration and deceleration circuit independent ofCPU program steps. No program software (e.g., firmware) overhead otherthan the initial step frequency profile table loading, and a startoperation are required.

Referring now to the drawings, the details of example embodiments areschematically illustrated. Like elements in the drawings will berepresented by like numbers, and similar elements will be represented bylike numbers with a different lower-case letter suffix.

Referring to FIG. 1, depicted is a schematic block diagram of athree-axis stepper motor circuit, according to the teachings of thisdisclosure. A three axis (x, y and z) stepper motor circuit, generallyrepresented by the numeral 100, CPU 132 and memory 130 are shown, butthe disclosure below also applies to two axis and single axis steppermotor circuits.

An NCO 126 may be used as a frequency prescaler and receives a clocksignal having a frequency, Fosc, from a clock oscillator 128 and aPrescale value, and provides a CNC_CLK signal having a frequency that isa function of the clock signal, Fosc, and the Prescale value. The NCO126 may be programmed with the Prescale value stored in a memory 130.The frequency of the signal CNC_CLK may be the frequency, Fosc, from theclock oscillator 128 divided by the Prescale value.

X-stepper motor 102 is driven by an X-step driver 108 at a step ratedetermined by the frequency of a signal from an X-NCO 120 which is afunction of the frequency of the CNC_CLK and an X-step rate programmedinto the X-NCO 120, e.g., from the CPU 132. An X-counter 114 may monitorthe number of step pulses (frequency cycles) from the X-NCO 120 andinhibit the X-NCO 120 from further output when a programmed count(#X-steps) is reached, e.g., programmed from the CPU 132. The output ofthe X-NCO 120 may be a function of the frequency of the input signal(CNC_CLK) from the prescaler NCO 126 divided by the X-Step Rate.

Y-stepper motor 104 is driven by a Y-step driver 110 at a step ratedetermined by the frequency of a signal from a Y-NCO 122 and a Y-steprate programmed into the Y-NCO 122, e.g., from the CPU 132. A Y-counter116 may monitor the number of step pulses (frequency cycles) from theY-NCO 122 and inhibit the Y-NCO 122 from further output when aprogrammed count (#Y-steps) is reached, e.g., programmed from the CPU132. The output signal frequency from the Y-NCO 122 may be a frequencyof an input signal (CNC_CLK) from the prescaler NCO 126 divided by theY-Step Rate. The NCO 126 receives a clock signal having a frequency,Fosc, from the clock oscillator 128 and outputs a signal with frequencyCNC_CLK as describe hereinabove.

Z-stepper motor 106 is driven by a Z-step driver 112 at a step ratedetermined by the frequency of a signal from a Z-NCO 124 and a Z-steprate programmed into the Z-NCO 124, e.g., from the CPU 132. A Z-counter118 may monitor the number of step pulses (frequency cycles) from theZ-NCO 124 and inhibit the Z-NCO 124 from further output when aprogrammed count (#Z-steps) is reached, e.g., programmed from the CPU132. The output signal frequency from the Y-NCO 122 may be a frequencyof an input signal (CNC_CLK) from the prescaler NCO 126 divided by theY-Step Rate. The NCO 126 receives a clock signal having a frequency,Fosc, from the clock oscillator 128 and outputs a signal with frequencyCNC_CLK as describe hereinabove.

A 45-degree angle movement will result when the step rates are the samefor the X and Y stepper motors 102 and 104. A different angle movementwill result when the X and Y step rates are different. The number ofstep pulses will determine how far the CNC machine will move whencontrolled by the stepper motor(s). Thus, keeping track of the number ofstep pulses from the X-NCO will determine the movement location of thetool controlled by the X-stepper motor 102, likewise for the Y andZ-stepper motors 104 and 106.

The Y-stepper and Z-stepper portions of the three-axis stepper circuitwork in substantially the same way as described above for the X-stepper.By programming unique X, Y and Z step rates into the X-NCO 120, Y-NCO122 and Z-NCO 124, respectively, movement of the CNC tool may beprogrammed to produce any pattern desired. In addition, the X-stepcounter (X-CNTR) 114, the Y-step counter (Y-CNTR) 116, and the Z-stepcounter (Z-CNTR) 118, may be programmed with the maximum number of stepsallowed for the X-stepper motor 102, Y-stepper motor 104, and Z-steppermotor 106, respectively, e.g., predetermined counts #X-Steps, #Y-Steps,#Z-Steps. If the maximum number of steps is reached then the stepcounter(s) (114, 116, 118) may inhibit step output from the respectiveNCO (120, 122, 124).

Referring to FIG. 2, depicted is a schematic block diagram of anautomated speed ramp control circuit using direct memory access (DMA)transfers to load speed ramp control values for acceleration anddeceleration of stepper motors, according to a specific exampleembodiment of this disclosure. A numerically controlled oscillator (NCO)126 is configured as a frequency prescaler with a clock input coupled toa clock signal having a frequency, Fosc, supplied from, for example butis not limited to, a clock oscillator 128, a prescale input coupled to aDMA control circuit 240, and an output supplying a CNC_CLK signal havinga frequency responsive to an input divisor value Prescale, for example,the frequency of signal CNC_CLK is the frequency Fosc divided by thePrescale value.

The NCO 126 will generate clock pulses, CNC_CLK, having a frequency thatis a function of the frequency Fosc divided by the Prescale value. ThePrescale value may be programmed into the NCO 126 by the DMA controlcircuit 240. The DMA control circuit 240 may comprise a direct memoryaccess (DMA) module 232, a data signal modulator (DSM) 234, and a pulsewidth modulator (PWM) module 238. The DMA module 232 may obtain thePrescale values from a memory 130. The memory 130 stores the Prescalevalues for both acceleration and deceleration of the stepper motor(s).The DMA module 232, may be for example but is not limited to a CIP, thatmay function independently from the CPU 132. Upon a reset to the DMAmodule 232, e.g., from the CPU 132, its address counter may be reset, afirst Prescale value is obtained from the memory 130 and programmed intothe NCO 126. This starts the NCO 126 which generates the clock pulses(CNC_CLK) at a frequency determined by the frequency, Fosc, from theclock oscillator 128 divided by the Prescale value programmed into theNCO 126.

The PWM module 238 (upon being reset) also starts at a first output (forexplanation purposes a logic low) which is received at a MOD input ofthe DSM 234. A logic low at the MOD input of the DSM 234 couples theCNC_CLK clock pulses to a trigger input of the DMA module 232. Each timea CNC_CLK clock pulse is received, the address counter in the DMA module232 will increment and the DMA module 232 will read a new Prescale valvefrom the memory 130 and transfer that new Prescale value into the NCO126 (DMA transfer). After a certain number of CNC_CLK clock pulses havebeen received by the PWM module 238 its output will go to a second logiclevel (for explanation purposes a logic high) and the output of the DSM234 may be decoupled from the CNC_CLK clock pulses and held at a logiclow, whereby no DMA transfers will occur. After a further number ofCNC_CLK clock pulses have been received by the PWM module 238 its outputwill go back to the first logic level, then the CNC_CLK clock pulseswill again be coupled to the trigger input (address counter) of the DMAmodule 232 and DMA transfers to the prescaler NCO 126 will resume forthe deceleration phase of the stepper motor speed profile. A PWM dutycycle for the PWM module 238 may be programmed from the CPU 132 by aDuty Cycle signal for any desired ratio of first and second logic levelsbased upon the number of CNC_CLK clock pulses to be received. Theaforementioned circuit and functions may be provided by amicrocontroller 200.

The DSM 234 may be configured to switch on the edge of the modulatedsignal. This prevents shortened pulses due to asynchronous timingbetween the modulation signal (output from the PWM module 238) and themodulated signal (CNC_CLK).

Referring to FIG. 3, depicted is a schematic block diagram of anautomated speed ramp control circuit using direct memory access (DMA)transfers to load speed ramp control values for acceleration anddeceleration of stepper motors, according to another specific exampleembodiment of this disclosure. The circuit shown in FIG. 3 functions insubstantially the same way as the circuit shown in FIG. 2 and describedhereinabove, with the addition of an accumulator counter 236 coupledbetween the output of the NCO 126 and the inputs of the PWM 238 and DSM234 modules. The accumulator counter 236 allows a certain number ofCNC_CLK pulses to occur before the next DMA transfer is initiated by theDMA module 232. For example, a plurality of CNC_CLK clock pulses wouldoccur before the next DMA transfer was initiated, responsive to anAccumulator Count Load input received from the CPU 132. Theaforementioned circuit and functions may be provided by amicrocontroller 300.

Referring to FIG. 4, depicted is a schematic block diagram of anautomated speed ramp control circuit using direct memory access (DMA)transfers to load speed ramp control values for acceleration anddeceleration of stepper motors, according to yet another specificexample embodiment of this disclosure. The circuit shown in FIG. 4functions in substantially the same way as the circuits shown in FIGS. 2and 3, and described hereinabove, with the addition of a counter 228.The counter 228 may be programmed with a maximum count, Max_Count,received from the CPU 132, that causes the prescaler NCO 126 to beinhibited from outputting further CNC_CLK clock pulses when theprogrammed maximum count is reached. It may also be used to reset thePWM module 238. This may prevent the CNC tool from exceeding a maximumtravel distance. The counter 228 may be reset by the CPU 132 upon thestart of a new acceleration/deceleration profile. The aforementionedcircuit and functions may be provided by a microcontroller 400.

Referring to FIG. 5, depicted are schematic graphs of the operation ofthe automated stepper motor acceleration and deceleration circuits shownin FIGS. 2, 3 and 4. An output waveform 540 from the PWM module 238 isshown in relation to the DMA transfers 542 (acceleration DMA transfers542 a and deceleration DMA transfers 542 b) and relative frequency ofthe CNC_CLK clock pulses 544. Initially, when the output 540 is at thefirst logic level acceleration DMA transfers 542 a are allowed, and whenthe output 540 is at the second logic level there are no DMA transfers542 (DMA initiation triggers to the DMA module 232 are inhibited). Thefrequency of the CNC_CLK clock pulses is determined by the NCO 126 basedon the Prescale values from the acceleration/deceleration table storedin the memory 230 and programmed into the NCO 126 during each DMAtransfer 542. Once the programmed Prescale value providing the highestfrequency of CNC_CLK clock pulses is reached, e.g., based upon thenumber of acceleration DMA transfers 542 a desired, further DMAtransfers are inhibited until a further number of clock pulses (CNC_CLK)occur, then the NCO 126 is again programmed with Prescale valuedeceleration DMA transfers 542 b to reduce the frequency of the CNC_CLKclock pulses for deceleration of the stepper motor(s).

It is contemplated and within the scope of this disclosure that thefunction of the DSM module 234 may be provided by a multiplexer or othercombination of logic functions known to one of ordinary skill in the artof digital electronics design and having the benefit of this disclosure.It is also contemplated and within the scope of this disclosure thatcounters and combinatorial logic may be used in place of the PWM module238. Referring to FIG. 6, depicted is a schematic block diagram offunctional circuit variations of the circuit shown in FIGS. 2, 3 and 4.For example, but not limited to, a DMA control circuit 640 may comprisea DMA module 232, a multiplexer 634, an acceleration counter 640, amaximum speed counter 642, a deceleration counter 644, an associatedlogic gates (not numbered).

The multiplexer 634 may be used in place of the DSM module 234 wherein alogic “0” at the cntl input will couple the CNC_CLK clock pulses to thetrigger input of the DMA module 232, and a logic “1” will inhibit theCNC_CLK clock pulses thereto. An acceleration counter 640 may count thenumber of CNC_CLK clock pulses during acceleration of the steppermotor(s), a maximum speed counter 642 may count the number of CNC_CLKclock pulses during maximum speed of the stepper motor(s), and adeceleration counter 644 may count the number of CNC_CLK clock pulsesduring deceleration of the stepper motor(s). Using combinatorial logic,the outputs of counters 640 and 642 may control input selection to themultiplexer 634. Counter 644 may provide an interrupt to the CPU 132 awhen deceleration of the stepper motors has finished. The aforementionedcircuit and functions may be provided by a microcontroller 600.

The present disclosure has been described in terms of one or moreembodiments, and it should be appreciated that many equivalents,alternatives, variations, and modifications, aside from those expresslystated, are possible and within the scope of the disclosure. While thepresent disclosure is susceptible to various modifications andalternative forms, specific example embodiments thereof have been shownin the drawings and are herein described in detail. It should beunderstood, however, that the description herein of specific exampleembodiments is not intended to limit the disclosure to the particularforms disclosed herein.

What is claimed is:
 1. A method for speed ramp control of a steppermotor using direct memory access (DMA) transfers, said method comprisingthe steps of: providing a numerically controlled oscillator (NCO) havingoscillator and prescale value inputs and an output providing clockpulses, wherein a frequency of the clock pulses is a function of theoscillator frequency and the prescale value; providing a table ofacceleration and deceleration prescale values in a memory; programmingthe NCO with the acceleration prescale values from the memory usingdirect memory access (DMA) transfers for a first number of clock pulseswithout requiring intervention or program execution by a centralprocessing unit (CPU); inhibiting DMA transfers to the NCO for a secondnumber of clock pulses; and programming the NCO with the decelerationprescale values from the memory using the DMA transfers for a thirdnumber of clock pulses without requiring intervention or programexecution by a central processing unit (CPU).
 2. The method according toclaim 1, further comprising the step of counting a certain number ofclock pulses from the NCO before doing the DMA transfer to the NCO. 3.The method according to claim 1, further comprising the step of countingthe number of clock pulses from the NCO then inhibiting the output ofthe NCO after the number of clock pulses reaches a maximum count.
 4. Anapparatus for automated speed ramp control of stepper motor accelerationand deceleration, comprising: an oscillator having an output; anumerically controlled oscillator (NCO) having oscillator and prescalevalue inputs and an output providing clock pulses, wherein the NCOoscillator input is coupled to the oscillator output and a frequency ofthe clock pulses from the NCO is a function of a frequency from theoscillator and the prescale value; a memory for storing a plurality ofacceleration and deceleration prescale values; and a direct memoryaccess (DMA) control circuit coupled between the NCO prescale valueinput and the memory, and the output of the NCO, wherein the DMA controlcircuit provides the NCO with the acceleration prescale values from thememory for a first number of clock pulses without requiring interventionor program execution by a central processing unit (CPU), thereafter theDMA control circuit is inhibited for a second number of clock pulses,and then the DMA control circuit provides the NCO with the decelerationprescale values from the memory for a third number of clock pulseswithout requiring intervention or program execution by a centralprocessing unit (CPU).
 5. The apparatus according to claim 4, furthercomprising an accumulator counter coupled between the output of the NCOand the DMA control circuit, wherein the DMA control circuit provides asingle prescale value to the NCO after a certain number of clock pulseshave occurred.
 6. The apparatus according to claim 4, further comprisinga maximum counter coupled to the output of the NCO and inhibitingoperation of the NCO after receiving a certain maximum number of clockpulses therefrom.
 7. The apparatus according to claim 4, wherein the DMAcontrol circuit comprises: a pulse width modulation (PWM) module havingan input coupled to the clock output of the NCO and an output; and adata signal modulator (DSM) having a first input coupled to the clockoutput of the NCO, a second input coupled to a fixed logic level, anoutput coupled to a DMA trigger input of the DMA module and a controlinput coupled to the PWM module output for selectively coupling theoutput of the DSM between the first and second inputs thereof, whereinthe first input is selected when the output of the PWM module is at afirst logic level and the second input is selected when the output ofthe PWM module is at a second logic level.
 8. The apparatus according toclaim 7, further comprising an accumulator counter coupled between theoutput of the NCO, and the first input of the DSM and the input of thePWM module, wherein the DMA module provides a single prescale value tothe NCO after a certain number of clock pulses have occurred.
 9. Theapparatus according to claim 7, wherein a duty cycle of the PWM moduleis programmable to provide the first logic level output during the firstnumber of clock pulses, the second logic level output during the secondnumber of clock pulses, and the first logic level output during thethird number of clock pulses.
 10. The apparatus according to claim 7,wherein the first logic level is a logic zero and the second logic levelis a logic one.
 11. The apparatus according to claim 7, wherein thefirst logic level is a logic one and the second logic level is a logiczero.
 12. The apparatus according to claim 7, wherein the output of theDSM is configured to switch between the first and second inputs thereofon an edge of a control signal to the control input of the DSM.
 13. Theapparatus according to claim 4, wherein the DMA control circuitcomprises: a first counter for counting the first number of clockpulses; a second counter for counting the second number of clock pulses;a third counter for counting the third number of clock pulses; and amultiplexer having a first input coupled to the NCO output, a secondinput coupled to a fixed logic level, an output coupled to the DMAmodule and input selection controlled by the first, second and thirdcounters; wherein the first counter controls the multiplexer to couplethe first input to the output thereof during the first number of clockpulses, the second counter controls the multiplexer to couple the secondinput to the output thereof during the second number of clock pulses,and the third counter controls the multiplexer to couple the first inputto the output thereof during the third number of clock pulses.
 14. Anapparatus for automated speed ramp control of stepper motor accelerationand deceleration, comprising: an oscillator having an output; anumerically controlled oscillator (NCO) having oscillator and prescalevalue inputs and an output providing clock pulses, wherein the NCOoscillator input is coupled to the oscillator output and a frequency ofthe clock pulses is a function of a frequency from the oscillator andthe prescale value; a memory for storing a plurality of acceleration anddeceleration prescale values; a direct memory access (DMA) modulecoupled between the NCO prescale value input and the memory; a pulsewidth modulation (PWM) module having an input coupled to the clockoutput of the NCO; and a data signal modulator (DSM) having a firstinput coupled to the clock output of the NCO, a second input coupled toa fixed logic level, an output coupled to a DMA trigger input of the DMAmodule and a control input for selectively coupling the output of theDSM between the first and second inputs thereof; wherein the DMA moduleprograms the NCO with the acceleration prescale values during a firstnumber of clock pulses without requiring intervention or programexecution by a central processing unit (CPU), does no programming of theNCO during a second number of clock pulses, and programs the NCO withthe deceleration prescale values during a third number of clock pulseswithout requiring intervention or program execution by a centralprocessing unit (CPU).
 15. The apparatus according to claim 14, furthercomprising an accumulator counter coupled between the clock output ofthe NCO, and the first input of the DSM and the input of the PWM module,wherein the DMA module provides a single prescale value to the NCO aftera certain number of clock pulses have occurred.
 16. The apparatusaccording to claim 14, further comprising a maximum counter coupled tothe output of the NCO and inhibiting operation of the NCO afterreceiving a certain maximum number of clock pulses therefrom.
 17. Theapparatus according to claim 14, wherein a duty cycle of the PWM moduleis programmable to provide the first logic level output during the firstnumber of clock pulses, the second logic level output during the secondnumber of clock pulses, and the first logic level output during thethird number of clock pulses.
 18. The apparatus according to claim 14,wherein the output of the DSM is configured to switch between the firstand second inputs thereof on an edge of a control signal to the controlinput of the DSM.